diff mbox

Make max_align_t respect _Float128 [version 3]

Message ID alpine.DEB.2.20.1609121744480.4945@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers Sept. 12, 2016, 5:45 p.m. UTC 
[Patch version 3 is updated for Jason's changes to
cxx_fundamental_alignment_p.]

The _FloatN, _FloatNx, _DecimalN and _DecimalNx types are specified in
such a way that they are basic types, meaning that max_align_t must be
at least as aligned as those types.

On 32-bit x86, max_align_t is currently 8-byte aligned, but
_Decimal128 and _Float128 are 16-byte aligned, so the alignment of
max_align_t needs to increase to meet the standard requirements for
these types.

This patch implements such an increase.  Because max_align_t needs to
be usable for C++ as well as for C, <stddef.h> can't actually refer to
_Float128, but needs to use __float128 (or some other notation for the
type) instead.  And since __float128 is architecture-specific, there
isn't a preprocessor conditional that means "__float128 is available"
(whereas one could test __FLT128_MANT_DIG__ to see if _Float128 is
available; __SIZEOF_FLOAT128__ is available on x86 only).  But I
believe the only case that actually has an alignment problem here is
32-bit x86, and <stddef.h> already has lots of conditional specific to
particular architectures or OSes, so this patch uses a conditional on
__i386__; that also is the minimal change that ensures neither size
nor alignment of max_align_t is changed in any case other than where
it is necessary.  If any other architectures turn out to have such an
issue, it will show up as failures of one of the testcases added by
this patch.

Such an increase is of course an ABI change, but a reasonably safe
one, in that max_align_t doesn't tend to appear in library interfaces
(rather, it's something to use when writing allocators and similar
code; most matches found on codesearch.debian.net look like copies of
the gnulib stddef.h module rather than anything actually using
max_align_t at all).

max_align_t_align has a corresponding change (adding _Float128 to the
types considered).

(I think glibc malloc alignment should also increase to 16-byte on
32-bit x86 so it works for allocating objects of these types, which is
now straightforward given the fix made for 32-bit powerpc.)

Bootstrapped with no regressions on x86_64-pc-linux-gnu, and
spot-tested with -m32 that the new float128-align.c test now compiles
where previously it didn't.  OK to commit?

gcc:
2016-09-12  Joseph Myers  <joseph@codesourcery.com>

	* ginclude/stddef.h (max_align_t) [__i386__]: Add __float128
	element.

gcc/c-family:
2016-09-12  Joseph Myers  <joseph@codesourcery.com>

	* c-common.c (max_align_t_align): Also consider alignment of
	float128_type_node.

gcc/testsuite:
2016-09-12  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/float128-align.c, gcc.dg/float128x-align.c,
	gcc.dg/float16-align.c, gcc.dg/float32-align.c,
	gcc.dg/float32x-align.c, gcc.dg/float64-align.c,
	gcc.dg/float64x-align.c, gcc.dg/floatn-align.h: New tests.

Comments

Joseph Myers Sept. 19, 2016, 3:58 p.m. UTC | #1 
Ping.  This patch 
<https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00652.html> is pending 
review.
Paul Eggert Sept. 19, 2016, 4:29 p.m. UTC | #2 
On 09/19/2016 08:58 AM, Joseph Myers wrote:
> Ping.  This patch
> <https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00652.html> is pending
> review.
>
Thanks, the patch looks good to me. It should be safe for the uses of 
max_align_t that I know of (e.g., Emacs).
Jeff Law Sept. 26, 2016, 4:27 p.m. UTC | #3 
On 09/19/2016 09:58 AM, Joseph Myers wrote:
> Ping.  This patch
> <https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00652.html> is pending
> review.
OK.

I never like adding more conditionals to stddef.h, but it's likely 
unavoidable in this case.

jeff
diff mbox

Patch

Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c	(revision 240091)
+++ gcc/c-family/c-common.c	(working copy)
@@ -12873,8 +12873,11 @@  scalar_to_vector (location_t loc, enum tree_code c
 unsigned
 max_align_t_align ()
 {
-  return MAX (TYPE_ALIGN (long_long_integer_type_node),
-	      TYPE_ALIGN (long_double_type_node));
+  unsigned int max_align = MAX (TYPE_ALIGN (long_long_integer_type_node),
+				TYPE_ALIGN (long_double_type_node));
+  if (float128_type_node != NULL_TREE)
+    max_align = MAX (max_align, TYPE_ALIGN (float128_type_node));
+  return max_align;
 }
 
 /* Return true iff ALIGN is an integral constant that is a fundamental
Index: gcc/ginclude/stddef.h
===================================================================
--- gcc/ginclude/stddef.h	(revision 240091)
+++ gcc/ginclude/stddef.h	(working copy)
@@ -426,6 +426,14 @@  typedef __WINT_TYPE__ wint_t;
 typedef struct {
   long long __max_align_ll __attribute__((__aligned__(__alignof__(long long))));
   long double __max_align_ld __attribute__((__aligned__(__alignof__(long double))));
+  /* _Float128 is defined as a basic type, so max_align_t must be
+     sufficiently aligned for it.  This code must work in C++, so we
+     use __float128 here; that is only available on some
+     architectures, but only on i386 is extra alignment needed for
+     __float128.  */
+#ifdef __i386__
+  __float128 __max_align_f128 __attribute__((__aligned__(__alignof(__float128))));
+#endif
 } max_align_t;
 #endif
 #endif /* C11 or C++11.  */
Index: gcc/testsuite/gcc.dg/float128-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float128-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float128-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float128 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float128 } */
+/* { dg-require-effective-target float128 } */
+
+#define WIDTH 128
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float128x-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float128x-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float128x-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float128 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float128x } */
+/* { dg-require-effective-target float128x } */
+
+#define WIDTH 128
+#define EXT 1
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float16-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float16-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float16-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float16 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float16 } */
+/* { dg-require-effective-target float16 } */
+
+#define WIDTH 16
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float32-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float32-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float32-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float32 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float32 } */
+/* { dg-require-effective-target float32 } */
+
+#define WIDTH 32
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float32x-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float32x-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float32x-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float32 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float32x } */
+/* { dg-require-effective-target float32x } */
+
+#define WIDTH 32
+#define EXT 1
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float64-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float64-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float64-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float64 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float64 } */
+/* { dg-require-effective-target float64 } */
+
+#define WIDTH 64
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float64x-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float64x-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float64x-align.c	(working copy)
@@ -0,0 +1,9 @@ 
+/* Test _Float64 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float64x } */
+/* { dg-require-effective-target float64x } */
+
+#define WIDTH 64
+#define EXT 1
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/floatn-align.h
===================================================================
--- gcc/testsuite/gcc.dg/floatn-align.h	(nonexistent)
+++ gcc/testsuite/gcc.dg/floatn-align.h	(working copy)
@@ -0,0 +1,18 @@ 
+/* Tests for _FloatN / _FloatNx types: test max_align_t alignment.
+   Before including this file, define WIDTH as the value N; define EXT
+   to 1 for _FloatNx and 0 for _FloatN.  */
+
+#define CONCATX(X, Y) X ## Y
+#define CONCAT(X, Y) CONCATX (X, Y)
+#define CONCAT3(X, Y, Z) CONCAT (CONCAT (X, Y), Z)
+
+#if EXT
+# define TYPE CONCAT3 (_Float, WIDTH, x)
+#else
+# define TYPE CONCAT (_Float, WIDTH)
+#endif
+
+#include <stddef.h>
+
+_Static_assert (_Alignof (max_align_t) >= _Alignof (TYPE),
+		"max_align_t must be at least as aligned as _Float* types");